Depends on what you're asking. If your question is which of the hero classes you should be choosing at level 2 for early/mid/late game strength, then my answer would be:
Early Game - Damage-line Warriors work well, as those +3 attack traits provide a very significant boost at this stage of the game, though I'd wait to see what high-end weapons I get before picking a weapon specialization. Tank-line Defenders work well, if you don't mind having your champion act mainly as a brick wall, but it requires some care with your supporting troops to hold the attention of the opposing army on your champion. Developing a Defender along the spell-resistance line has some nice active abilities, but the passive bonuses are not generally useful. Assassins are okay but they rely on getting critical hits whereas Warriors just have higher base damage, and dodge is a little unreliable for durability. Damage Mages are weak because you probably don't have the good spells yet or a good mana supply for casting them, but summoner Mages are at their most useful, if you can get enough mana income to maintain strategic summons and keep them alive through a few battles. Support Mages are kind of in the same boat as damage Mages, but if you have Haste or Slow and pick targets well, they can still be very useful, as long as you keep a close eye on your mana reserves. Governor Commanders are useful, and depending on how quickly you level them and what you pick for them could represent a fairly considerable portion of your research or gold income, while Field Commanders provide a mediocre passive boost to your army and have a decent active ability accessible early.
Mid Game - Direct combat champions (Warriors, Defenders, and Assassins) are beginning to be eclipsed by trained troops. If you're lucky and find a good weapon in loot and buy or find a full set of leather armor, then these will still work well, but to keep a front-line champion on the front line you're going to want weapons which would qualify for late game troops (pikes, great axes, longswords, longbows) or you need to find a late-game champion weapon (Curgen's Hammer, the lightning bow that gives +2 damage per level, a few others), and at least for Defenders you're going to want to at least consider getting heavier armor. If you can't find a good weapon for a Warrior or an Assassin, then it's time to start considering using them more as a mage or archer than a part of the front line (I would suggest that using them as a support mage is better than as an archer, as a four or five figure unit of archers with even the first bow is likely to be superior to any archer champion, as my luck in getting the quest that awards an Ignys Shortbow is poor, and bows and ranged staves are rather rare in loot). Governor Commanders who developed the income or research traits should begin to be eclipsed by towns, while if they developed the Administration line you should be moving to the point where you have enough cities that parking a champion for unrest reduction is becoming valuable. Field Commanders should be granting enough of an accuracy bonus for your troops to almost always hit, but if you've got a decent fortress up (and by now, you probably should), the accuracy bonus is kind of unnecessary. The initiative bonus and the active ability, however, are very useful, and you should start trying to reach the other active ability you have access to, as your army should be large enough for this to be a very strong ability. Summoner Mages should begin switching from running around with strategic summons in their army to running around with trained troops, supported by tactical summons, as strategic-level summons don't really seem to scale into the mid and late game very well and you should now have enough of a mana income that you can summon at least one or two creatures per battle. Damage Mages depend on what spells you have access to, but they are starting to become strong. Support Mages should now have the mana available for casting curses upon entire armies of enemies or blessings on your entire army in almost every battle, and possibly doing both. Note though that you probably still need to be careful with your mana supply as any type of mage at this stage.
Late Game - Unless you found good equipment, all of your Warriors, Assassins, and Defenders should be used like archers or mages. If you were unable to find good equipment, then trained troops probably outclass your champions by so much that they can probably knock them out in one or two hits. If you found the kinds of equipment that late-game troops use, then you might be able to keep the champions on the front line for a little while, but they won't be able to deal as much damage as a late game unit of front line troops, and they cannot take hits as well either. All kinds of mages are fairly good at this point, although if your enemy has archers and you can't kill them before they act, your mage will probably fall, so make sure your mage has high initiative (if possible, buy your mage all the initiative-granting equipment they can wear in the shop despite the cost). Summoner and support mages should start branching out into damage spells, and summoners should look into support spells, since only the late-game summons are really worthwhile in battle anymore and there are only so many friends to bless and enemies to curse, and you should have plenty of mana. Field Commanders with the active that makes the whole army go are great, and the Command ability is still useful, but aside from that they should be looking into support and damage spellcasting or perhaps archery (but again, bows and ranged staves seem to be hard to find in loot) since they cannot compete directly with the troops they lead.
If you were asking which sovereign profession was best in each phase of the game, then:
Beastlords are good early and mid game if you develop them as curse spellcasters - tame needs to overcome enemy spell resistance to work - but start to lose out late game, as there are not a lot of beasts that are really able to combat late-game troops on equal terms, and the ones which can tend to be difficult to find, few in number, and hard to tame.
Bandit Lords are mostly good at the early game, as even by the late part of the early game your trained troops should completely outclass any bandits you can find. The starting units are nice, but nothing extraordinary.
Warlords are more mid and late game, as they essentially allow you to maintain two armies for every one you'd normally have. The bonus is still very useful in the early game, though, as you can field your normal early armies with a much stronger economy than you'd otherwise have, or you can keep your normal economy and have about twice the army.
Generals give no direct bonuses, but the longer you can keep them and their army alive, the better they and their army become.
Summoners are all about the early game. Shadow Wargs are virtually useless mid- and late-game, and the +2 level bonus to summoned creatures isn't bad, but it isn't great either.
Noble is always useful, but more so when your empire is large, so they're more a mid or late game profession.
Adventurer is more of an early-game bonus, though it might also tank your economy if you use it to get lots of champions too quickly.
Warlocks are for the mid and late game, when you have potent damage spells and the mana supply to be constantly casting them.
Hunters are for the early game, because extra damage against beasts isn't very useful at any other time.
Diplomats, in my opinion, are never useful, but that's more a result of my dislike of the game's diplomacy mechanics. I would expect them to be more useful in the mid and late game than in the early game.
I can't recall what other options do, if I've missed any.
Also note that this is my opinion, based on how I play. What's fun for me might not be fun for you, so I suggest experimenting to see what you like.